Solar energy collector

ABSTRACT

Disclosed are structures for collection and concentration of solar energy, each including one or more longitudinally extending, cusp-like modules made up of apposed reflectors of any desired length. The reflectors comprise ruled surfaces and are arranged to converge from entrance to exit apertures. One reflector in each module is concave, and the modules are asymmetric in section, although two asymmetric modules may be combined in a system which is itself symmetrical. A pair of modules may be combined to have a common plane reflector, and the pairs may be further combined if desired. The modules are supported and oriented according to the latitude of the installation, to obtain intense solar energy concentration, which is accomplished totally without diurnal tracking.

This is a division of U.S. application Ser. No. 295,749, filed Aug. 24,1981, now U.S. Pat. No. 4,444,176, which is a division of U.S.application Ser. No. 12,838, filed Feb. 16, 1979 and is now issued asU.S. Pat. No. 4,297,988, Nov. 3, 1981.

BACKGROUND OF THE INVENTION

This invention relates to the field of optical engineering, andparticularly to the design of efficient, costeffective solar energycollectors. Sunlight falling on the earth represents a source of radiantenergy, even though at any location on the earth it is not continuous,being cyclically interrupted in accordance with the diurnal rotation ofthe earth.

Radiant energy from the sun reaches the unclouded earth at a maximumflux density of about 1000 watts per square meter of surface impingedperpendicularly. Solar energy at this natural intensity is suitable fornumerous purposes, including lighting and photosynthesis, but is not ina form readily usable for a large number of other applications.

Much thought has been devoted to developing apparatus for convertingsolar radiant energy to other forms of energy which are more generallyuseful. Devices for performing this function are known as solarcollectors, and simple collectors may do no more than accept radiantenergy at its natural intensity and enable it to fall on absorbentmaterial, thereby converting it to thermal energy which is then used toraise the temperature of a suitable, usually liquid, heat transportmedium. Flow of the medium transports the heat energy to a more or lessdistant point of use. A typical apparatus of this sort is known as aflat-plate collector. The problems caused by periods of darknessintervening between periods of insolation, and by the frequent presenceof cloud cover, are common to all solar energy collection systems, butare not addressed as a part of the present invention and will not beconsidered further here.

For many applications, the flux density of solar radiant energy isinadequate: efficient power generation with superheated stem is oneexample. To meet the demands of such applications, solar collectors havebeen designed which increase the effective flux density by concentratingthe energy incident at an entrance aperture of a first area, so that itis directed to an exit aperture of considerably smaller area where anenergy absorber or other useful receiver is located. For example, if theenergy at normal intensity reaching an entrance aperture thirtycentimeters wide is concentrated to reach an exit aperture threecentimeters wide and of the same length, the energy at the exit aperturehas a flux density ten times as great as that at the entrance aperture,assuming no loss of energy during concentration. In this discussion itis to be understood that the effective size of an aperture is notmeasured directly by its physical dimensions, but rather by thecomponents of those dimensions normal to the direction of incidentenergy, that is, to the "sun line".

The direction of the sun from any point on the earth's surface is notconstant. It varies both in altitude and in azimuth from sunrise tosunset, and it also varies seasonally. At 40° north or south latitudethe sun's altitude varies from 0° at sunrise to a maximum angle of 73.4°at noon of the summer solstice.

Any square meter of collector surface fixed to the earth, even ifperpendicular to the sun line at a particular time of a particular day,is for most daylight hours not perpendicular to that line, and thuseffectively represents less than a square meter of entrance aperture.With respect to this problem, solar energy collectors may be dividedinto two categories defined broadly as tracking collectors andnon-tracking collectors, both categories being capable of design toconcentrate the radiant flux density.

A tracking collector is one which is mounted for movement with respectto the earth's surface, and in which the collector is moved by a diurnaltracking mechanism to keep it pointed directly at the sun as itapparently moves through the heavens, so that the entire aperture isalways perpendicular to the sun line from morning to night, and hence isalways of maximum effective area. The requirement for tracking movementof the collector obviously places practical limitations on the area ofthe collector.

A non-tracking collector is one which is fixed to the earth, and istherefore subject to diurnal change in effective entrance aperture. Itis not, however, subject to practical limitations as to physical size,which therefore becomes limited only by the area available for use andthe cost of materials and labor. A typical non-tracking collectorcomprises one or more sets of elongated reflectors each concentratingenergy from the sun on an elongated narrow receiver such as a waterpipe. The reflectors and receivers extend east and west, and may beconstructed in banks oriented at an angle which deviates from thehorizontal in accordance with the latitude of the location.

In a tracking collector the upper limit of concentration is accomplishedwhen all the radiant energy received at the entrance aperture is focusedor imaged at the exit aperture, where a suitable energy receiver ispositioned. Application, to non-tracking collectors, of the opticalprinciples of focusing of energy does not give promise of sufficientlygreat multiplication of the flux density from entrance to exitapertures.

It is possible, however, to design a solar collector that concentratesthe rays of the sun but does not focus them. Such a device is called anon-imaging collector, and to forego imaging is to gain a degree ofdesign freedom that can be put to useful ends.

Non-imaging, non-tracking collectors are not without problems, however.First, a collector of this sort for full coverage must be designed withan acceptance angle at least as great as the range of the sun's altitudeangle, or some of the radiant energy will escape collection. Second,reexit of rays which have entered the collector must be prevented, assuch rays are lost to the collector and reduce the flux density at theexit aperture.

SUMMARY OF THE INVENTION

The present invention is directed to non-imaging non-tracking solarenergy collectors having energy reflectors of improved design wherebyreexit of energy which has passed in through the entrance aperture issubstantially reduced, or in some embodiments is eliminated. The designincludes pairs of reflectors of particular curvature, orientation, andapposition. The reflectors are elongated in an east-and-west direction,and are configured as convergent surfaces comprising apposed portions ofintersecting ruled surfaces defined by parallel generatrices, not morethan one of the surfaces being concave, and the entrance apertures beingarranged to give acceptance angles which match the apparent localaltitude range of the sun from the horizon to its zenith.

Several embodiments of the invention are shown, including pairs ofreflectors whose apposed surfaces are convex and concave conic sectionsor convex and concave spirals. Also included are pairs of surfaceshaving sections one of which is linear and the other of which isconcave: these pairs can be grouped with their plane surfaces incoincidence, and the groups themselves can be further combined.Arrangements are also disclosed in which one of the surfaces may insection be complex rather than a single curve, or may be discontinuous.Common to all these arrangements is the characteristic that the workingsurfaces of any pair are converging and asymmetrical. It is intendedthat a plurality of such reflective pairs or groups may be assembled ina properly oriented bank of any desired length, according as the energydemands of the application required.

Various advantages and features of novelty which characterize myinvention are pointed out with particularity in the claims annexedhereto and forming a part hereof. However, for a better understanding ofthe invention, its advantages, and objects attained by its use,reference should be had to the drawing which forms a further parthereof, and to the accompanying descriptive matter, in which there isillustrated and described a preferred embodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawing,

FIG. 1 is a schematic showing of a generalized collection moduleaccording to my invention;

FIG. 2 is a fragmentary schematic showing of a solar collector usingmodules according to FIG. 1;

FIG. 3 gives details of the configuration of the reflectors for onemodule according to the invention, using surfaces of parabolic section;

FIG. 4 is a view similar to FIG. 3 using surfaces of spiral section;

FIG. 5 is a sketch illustrative of the manner of loss of energy inconcentrators using plane reflectors;

FIG. 6 shows how a module according to the invention having a desiredentrance aperture may have either of two spatial orientations;

FIG. 7 shows a collector in which two pairs of reflectors share a planesurface;

FIG. 8 shows how two collectors of the type shown in FIG. 7 can becombined to advantage;

FIG. 9 shows a module in which one reflector is in section a complexcurve;

FIG. 10 shows that one reflector may be in section a composite of threeparabolas;

FIG. 11 shows that for large installations a reflector may beconstructed of arcs of a single parabola translated in space in analogyto a Fresnel lens.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A generalized module 20 for solar collectors embodying my invention isshown in FIG. 1 to comprise first and second reflectors 21 and 22 havingthe form of ruled surfaces and so configured that they intersect along aline 23, or would so intersect if extended. The apposed inner surfaces24, 25 are those which receive solar energy at an entrance aperture 26and by reason of their special configuration direct it to an exitaperture here represented as the site of a conduit 27 for liquid to beheated. A module is to be positioned with the line 23 extendinghorizontally east and west: the compass directions shown in FIG. 1 arefor northern hemisphere operation. It is clear that the length of such acollector is limited solely by the available space and the cost of laborand materials for its construction. The gain in flux density is not highin this structure. It is, however, admirably adapted to assembly in abank of such structures, as is suggested in FIG. 2, and such a bank ofsuch structures, as is suggested in FIG. 2, and such a bank of coursecan also be as long as desired.

A cross section of collector 20 perpendicular to line 23 has theconfiguration of a rhampoid cusp, and it is a characteristic of myinvention that only one of the reflectors can be concave: the other mustbe plane or convex. FIG. 3 shows how such a cusp 28 can be defined by apair of cylindrical paraboloids, and FIG. 4 shows a cusp 29 defined by apair of cylindrical spiraloids.

Attention is first directed however, to FIG. 5, which is supplied toillustrate the loss of energy entering a collector 20' at an entranceaperture 30 between plane reflectors 31 and 32 for collection at an exitaperture 33. The path of a typical ray 34 is traced, and it is seen thatafter multiple reflections, the ray direction reverses, to re-exit fromthis collector. Thus, for exit aperture 23, ray 34 lies outside theacceptance angle of the solar collector 20'. The acceptance angle of 20'may be increased by widening the angle between reflectors 31 and 32, orby moving exit aperture 33 toward entrance aperture 30, but in bothcases concentration will be lessened. The same effect may be noted incollectors where both of a pair of apposed reflectors are concave, andin imaging collectors when not aimed at the sun.

Unless otherwise stated, in the discussions which follow, it will beassumed for illustrative purposes that the collector is being designedfor use at 40° north latitude, where the solar altitude varies from 0°to a maximum of 73.4°. In the drawing β will be used to identifylatitude angle, and α will be used to identify the maximum solaraltitude angle, which of course is different for different latitudes,and may, near the equator, be more than ninety degrees.

FIG. 3 shows a cusp 28 defined by a pair of identical parabolic arcs 34and 35 having, respectively, foci 36 and 37 spaced along a line 38making the latitude angle with the horizontal, horizontal axes 40 and41, and directrices 42 and 43, so that they intersect at point 44. Arc34 continues from point 44 to a point 45, and arc 35 continues frompoint 44 to a point 46. Points 45 and 46 define the ends of the entranceaperture 47 of this cusp, which preferably makes the latitude angle βwith respect to the horizontal. One way to accomplish this is to sochoose points 45 and 46 that the tangents 48 and 49 to the respectivecurves at these points both make with the horizontal an angle α/2 equalto half the maximum solar altitude.

A typical ray 50 entering this cusp is traced on the curve throughsegments 51, 52 and 53 toward an exit aperture 54, where an appropriateenergy receiver may be located.

A further point should be made at this time. The solar energy reachingthe earth at dawn, and at dusk, is considerably attenuated by itsincreased path through the atmosphere. For some applications it may bewiser to reduce the acceptance angle and reposition the collector sothat it collects between say 15° and 73.4° altitude: such a modificationcould result in more effective conversion of the solar energy during thehours when atmospheric attenuation is least, and hence result in anoverall improvement in the conversion, the reduction in hours being morethan compensated by greater energy output during the shorter period.

The similar option is available of constructing the collector so that αis the maximum solar altitude during the winter, if the application issuch that summer use is not intended. Here again an increasedeffectiveness during the actual period of use may more than compensatefor the accompanying loss in generality.

FIG. 6 is presented to show schematically that a cusp having an entranceaperture 47 may be constructed of apposed concave and convex curves, tocurve either downwardly, as shown in solid lines, or upwardly, as shownin broken lines, the exit apertures 58 and 59 respectively being alsoshown.

FIG. 4 schematically shows a cusp 29 defined by a pair of arcs 55 and 56of Archimedian spirals which intersect at point 57. The origin forspiral 55 is at 0; the curve crosses the X-axis of origin O at 0',spaced from 0 by a distance a, and its radius vector r₁ in the firstquadrant about 0 has the value

    r.sub.1 =a+k.sub.1 1                                       (1)

The origin for spiral 56 is at P; the curve crosses the X-axis of originP at P', spaced from P by a distance c, and its radius vector r₂ in thefirst quadrant about P has the value

    r.sub.2 =c+k.sub.2 2.                                      (2)

In considering these two equations, c is greater than a, and k₂ isgreater than k₁ : the axes of ordinates through origins O and P arevertical and coincide, and P is below O. The curves are continued topoints 60 and 61, and the line 62 joining these points is the entranceaperture: as in FIG. 3, the entrance aperture 62 preferably makes thelatitude angle with the horizontal. The tangents 65, 66 to the curves atpoints 60 and 61 make, with the horizontal, angles equal to half thedifference between the latitude angle and the maximum solar angle, thatis (β-α)/2

Energy entering aperture 62 progresses by multiple reflection to an exitaperture 63, where an appropriate energy receiver may be located.

The cusps shown in FIGS. 3 and 4 are of course, cross sections ofmodules as exemplified in FIG. 1, and can be extended indefinitely in adirection perpendicular to the section plane. In each the energy entersthe space between convex and concave reflectors. Attention is nowdirected to FIG. 7, which illustrates in section a collector 69 of twocusps 70 and 71. The first cusp includes a concave parabolic reflector72 above a plane reflector 73, while the latter cusp includes a concaveparabolic reflector 74 below a plane reflector 75 which is in fact theopposite surface of reflector 73. Reflector 72 comprises a parabolic arcof which the focus is at 76 and the directrix is at 77, the axis 79 ofthe parabola being horizontal. Reflector 74 is a parabolic arc of whichthe focus is also at 76, but the axis 80 of the parabola makes with thehorizontal an angle α equal to the maximum solar altitude: the directrixof arc 74 is at 81, and makes with directrix 77 an angle equal to thesupplement of angle α. The arcs intersect at point 82, and the exitapertures of both cusps are defined by the line 76-82 which lies in theplane of reflectors 73 and 75. Curves 72 and 74 terminate at points 83and 84, their respective intersections with axes 80 and 79 respectively.These points are so located that a horizontal line 85 through point 83intersects the plane of reflectors 73 and 75 at the same point 86 asdoes the line 87 passing through point 84 at an angle α, equal to themaximum solar altitude. The plane of reflectors 73 and 75 then makeswith the horizontal an angle α/2 equal to half the maximum solaraltitude angle. Bidirectional energy receiving means may be located atthe line 76-82.

It should be pointed out at this time that while the preferred terminalpoints of various reflectors have been specifically recited inconnection with FIGS. 3, 4 and 7, variations in their location to modifythe entrance apertures and acceptance angles of the cusps may affect theflux density provided by the collectors, but not their operativeness.

The entrance aperture of cusp 70 is line 85, and that of cusp 71 is line87. Each cusp has its individual acceptance angle, but the two overlapso that the joint acceptance angle extends from 0 to α degrees. Mystudies have shown that, assuming perfect reflection from specularsurfaces, a pair of cusps as just described concentrate all of thedirect solar radiation impinging upon apertures 85 and 87 and direct ittotally upon the collection surfaces lying between point 76 and 82without loss throughout the day and throughout the year.

This improvement can be carried forward, with the same total efficiencyand with greater concentration, as suggested schematically in FIG. 8,which shows a pair of collectors 90 and 91 each generally like acollector 69 of FIG. 7. Collector 90 is of two cusps 92 and 93,comprising a central plane, bilateral reflector 94 between upper andlower concave reflectors 95 and 96 comprising portions of parabolashaving a common focus 90' and axes 95' and 96' respectively: its exitaperture is shown at 97. Collector 91 is of two cusps 100 and 101comprising a central plane bilateral reflector 102 between upper andlower concave reflectors 103 and 104 comprising portions of parabolashaving a common focus 91' and axes 103' and 104' respectively: its exitaperture is shown at 105. Planes 94 and 102 intersect at a dihedralangle α/2, and the bisector 106 of this angle makes an angle α/2 withrespect to the horizontal. Axis 103' is horizontal, axis 96' is at themaximum altitude angle α, and axes 95' and 104' are parallel withbisector 106. In one embodiment of the invention arcs 95 and 96terminate at points 110 and 111, at which tangents 110' and 111' tocurves 95 and 96 are parallel to the plane of reflector 94, and arcs 103and 104 terminate at points 113 and 114, at which tangents 113' and 114'to curves 103 and 104 are parallel to the plane of reflector 102. Points110 and 111 lie in a horizontal line 112, and points 113 and 114 lie ina line 115 which makes the maximum solar altitude angle α with respectto the horizontal, lines 112 and 115 intersecting bisector 106 at acommon point 116. Reflectors 94 and 102 terminate at lines 112 and 115respectively. In another embodiment of the invention arcs 95 and 104 maybe terminated nearer their respective foci, as at points 117 and 118,respectively.

Further improvement in concentration is possible with furthersubdivision into more collectors than two, and by modification of thereflecting surfaces as will be discussed in connection with FIGS. 10 and11.

FIG. 9 is a sectional view of a collector in the form of a modified cusp120 made up of a plane reflector 121 and a concave reflector 122 whichis a composite of a parabolic arc 123 and a composite circular andspiral arc 124. Arc 123 has a focus 125, a directix 126, and ahorizontal axis 127. The entrance aperture 128 of the collector isbetween point 130 on reflector 123 and point 131 on reflector 121, onlythe upper surface of which is utilized. The exit aperture in thiscollector is occupied by an energy receiver 132.

FIGS. 10 and 11 are presented to suggest further embodiments which myinvention may take. FIG. 10 shows a sectional view of a cusp 140 havinga plane reflector 141 and a generally concave reflector 142 made up ofarcs 143, 144 and 145 of three parabolas. The focus of arc 143 is atpoint 146, in the plane of reflector 141, as has been previouslydescribed, and the exit aperture of the collector is shown at 147. Thefoci of arcs 144 and 145 are at points 148 and 149, selected at thediscretion of the designer. I have found that this modification of thestructure may in some instances result in a greater flux density atreceiver 147 than is available from a single curve.

Finally, FIG. 11 is presented to show that it is not necessary for theconcave reflector to be continuous. The basis for the reflector 151 ofthis figure is a parabola 152 with a focus 153, a directrix 154 and ahorizontal axis 155. The actual reflector is made up of segments 156,157, 158, 159, 160, 161, 162, 163, 164, 165, 166 and 167. Segment 161coincides with parabola 152. The other segments may be spaced laterallyfrom parabola 152, either inwardly or outwardly, in what may beconsidered an analogy to the well known Fresnel lens. It is understoodthat the portions of the broken line 156-167 between the numberedsegments have no optical effect: they may be construction members oreven may be omitted if this is appropriate. The solar energy falling onthem is not collected, but the purpose of my invention here is not togather as much energy as possible, but rather to increase the energydensity at an exit aperture as much as possible. Segments 156-167 may beplanes positioned to approximate the paraboloid without unacceptableloss of concentration, a possibility which may be advantageous ifreflectors of large dimensions are to be constructed of simplecomponents.

From the above it will be evident that I have invented a new andimproved solar collection module, made up of a pair of asymmetricalreflectors converging from an entrance aperture to an exit aperture, andcomprising apposed portions of reflecting surfaces not more than one ofwhich is concave. The surfaces may be geometrically defined as ruledsurfaces defined by parallel generatrices, and may be paraboloidal orspiraloidal: only one of the reflectors may be plane. In a very usefulform of the invention the modules are used in pairs, with two planereflector surfaces coincident. The structure is well adapted forinstallations of large dimensions, either linearly or in area.

Numerous characteristics and advantages of my invention have been setforth in the foregoing description, together with details of thestructure and function of the invention, and the novel features thereofare pointed out in the appended claims. The disclosure, however, isillustrative only, and changes may be made in detail, especially inmatters of shape, size, and arrangement of parts, within the principleof the invention, to the full extent indicated by the broad generalmeaning of the terms in which the appended claims are expressed.

What is claimed is:
 1. A stationary solar collector fixed with respectto the surface of the earth at a latitude where the maximum solaraltitude angle is α, comprising:an elongated bidirectional energyreceiver affixed to the earth and lying along a line substantially inthe east-west direction; a first planar reflector affixed at an anglenot greater than α/2 relative to the surface of the earth and having twoplanar reflective surfaces extending from said energy receiver to apredetermined first end; a second elongated and curved reflector havinga parabolic cross section and having a concave reflective surface facingone of said planar reflective surfaces, said second reflector having afocal line lying in a plane aligned with said energy receiver, andhaving a parabolic surface length defined by a first line ofintersection with a planar extension of said first reflector, and asecond line of intersection with a plane constructed from said firstreflector first end, said constructed plane having a maximum inclinationdefined by the horizontal; and a third elongated and curved reflectorhaving a parabolic cross section and having a concave reflective surfacefacing the other of said planar reflective surfaces, said thirdreflector having a focal line lying in a plane aligned with said energyreceiver, and having a parabolic surface length defined by said firstline of intersection with a planar extension of said first reflector,and a second line of intersection with a plane constructed from saidfirst reflector first end at an angle not greater than α relative to thesurface of the earth.
 2. The apparatus of claim 1, wherein said secondand third reflectors each respectively describe in cross section,segments of parabolas intersecting one another, each of said segments ofparabolas lying along a generally arced line.
 3. The apparatus of claim1, wherein the respective parabolic cross sections of each of saidsecond and third reflectors are formed from discontinuous segments of asingle parabolic curve.
 4. The apparatus of claim 1, further comprisingeach said concave reflective surface respectively describing in crosssection discontinuous segments of a single parabola, said segments beingspaced laterally either inwardly or outwardly from a single parabola.